The present invention relates to the conversion of a glass sheet bending and tempering operation to a flat glass tempering operation. In particular, the invention deals with the modification of a conventional type of bending and tempering facility wherein a series of glass sheets are conveyed in a generally horizontal disposition through a heating chamber, bent to a desired curvature in a shaping station, and then rapidly cooled by blasts of tempering medium in a quenching station so as to impart a temper to the glass. A particular example of such a bending and tempering installation is disclosed in U.S. Pat. No. 3,701,644 which employs vertically reciprocating sets of shaping rolls for bending the glass sheets as they are continuously conveyed in a horizontal direction. It is occasionally desired to use the heating chamber and quenching means of such an installation to temper flat glass sheets without bending. The possibility of such an alternate use for a bending line is suggested in the above-mentioned U.S. Pat. No. 3,701,644, since in that arrangement straight conveyor rolls are provided in the bending zone, as well as the shaping rolls. However, in actual practice when flat glass sheets are conveyed through a shaping zone as taught by said patent, it has been found that the tempered glass sheets thus produced are often warped to such an extent that they are unacceptable for flat glazing applications. Thus, it has been highly desirable to find a way in which a horizontal glass sheet bending line could be economically used to temper flat glass sheets.
U.S. Pat. No. 3,245,772 to J. H. Cypher et al. discloses the imposition of unequal thermal conditions from the top surface to the bottom surface of a glass sheet in a heating furnace in order to deliberately bow the sheet as it passes through the furnace. Subsequently, in the quench zone an air pressure differential between top and bottom surfaces is maintained in order to flatten the sheet. Hold-down rolls are shown in the quench which press against the upper surface of a glass sheet passing therethrough.
In U.S. Pat. No. 3,801,298 to Bezombes there is shown an arrangement for bending and tempering glass sheets wherein the quenching zone includes curved rolls above and below the curved glass sheets being conveyed therethrough. The patent does not deal with the problem of maintaining the flatness of flat glass sheets being tempered.
U.S. Pat. No. 3,992,181 to Frank discloses application of air to balance the cooling rate of glass sheets being bent in a roll pressing operation.